It is generally known to the public that recent audio systems enable passengers to control the balance of volume between the front and rear speakers, which are located in the front and rear ends of the passenger compartment room of the automotive vehicle, respectively. Such systems are shown, for example, in Japanese Laid-open Patent Application No. 61-12310.
Usually, such an audio system can be controlled to lower the volume of the rear speakers when a person who does not want to listen to loud sounds, like a sleeping baby, is in the rear seat. On the other hand, the audio system can be controlled to lower the volume of front speaker when the person in a front seat likes to listen to stereophonic sound.
These controls are calibrated to attenuate equally all the bands from low frequency to high frequency. However, when the front speakers are turned off, passengers in the front seat experience difficulties hearing low frequency sounds. This difficulty is caused by the nature of low frequency sound, which is inherently more difficult to perceive than high frequency sound. Therefore, when the front speakers are turned off it is more difficult to discriminate the low frequency sounds.